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Xylanase pretreatment leads to enhanced soda pulping of wheat straw
Enzyme and Microbial Technology 30 (2002) 734 –740
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Xylanase pretreatment leads to enhanced soda pulping of wheat straw Jian Zhao,* Xuezhi Li, Yinbo Qu, Peiji Gao State Key Laboratory of Microbial Technology, Shandong University, Jinan 250100, People’s Republic of China Received 15 June 2001; received in revised form 08 November 2001; accepted 26 November 2001
Abstract Wheat straw was treated with a crude enzyme containing mainly xylanase prior to soda cooking. The enzyme was prepared from the culture filtration of Aspergillus niger strain An-76. The suitable conditions of pretreatment were xylanase dosage of 4 IU/g (on oven dry wheat straw), 48°C, 6 – 8 h, and pH 5.0, pretreated wheat straw was thorough washed after pretreatment. Xylanase pretreatment can improve pulpability of wheat straw effectively, and produce the pulps with lower kappa number (about 2 units lower), lower rejects and similar yields of screened-pulps under identical pulping conditions. The residual active alkali consistency of black liquor was increased by xylanase pretreatment. The brightness of bleached pulps from the xylanase pretreated wheat straw was about 3%ISO higher than that of the controls using identical bleaching sequence and conditions. The pulps, including unbleached pulps and bleached pulps, from the xylanase pretreated wheat straw had a higher tear index, a lower tensile index, and a lower burst index as compared to the control. The changes in the chemical components of wheat straw, crystallizability degree of cellulose, fiber average length and fines contents in the pulps were investigated in order to interpret differences of pulping results from the xylanase pretreated wheat straw and the controls. © 2002 Elsevier Science Inc. All rights reserved. Keywords: Xylanase pretreatment; Soda pulping; Aspergillus niger; Wheat straw
1. Introduction At present, wheat straw is an important raw material for pulp and paper manufacture and soda process is one of the main pulping processes to produce chemical pulp from wheat straw in the pulp and paper industry in China. However, the conventional soda process causes a serious wastewater pollution problem. In recent years, increasing environmental pressures and more rigorous regulations on strengthening the environmental protection have made it necessary to search for alternative methods for reducing wastewater pollution from pulping and bleaching plant in pulping industry. These pressures have also led to the design of many techniques aimed at improving the pulping process and reducing the lignin content of the pulp entering bleach plant. Lowering the kappa number of pulp by increasing the efficiency of the pulping process has led to a reduction in the amount of chemical reagents necessary for pulp bleaching and a concomitant reduction in the pollutants discharged from bleach plant. * Corresponding author. Tel.: ⫹86 531 8564429; fax: ⫹86 531 8565234 E-mail address: [email protected] (J. Zhao).
During the last decade, a large number of studies have been carried out on biopulping and biobleaching, especially biomechanical- and biochemical-pulping of wood (including softwood and hardwood) involving the use of lignin degrading fungi (i.e. white rot fungi) to treat wood chips and the use of xylanases to treat pulps prior to conventional pulping and bleaching [1–12]. Fungi pretreatment might reduce the refining energy and chemicals consumption of pulping processes or improve pulps properties, and reduce the negative environmental impact of pulping. But the main disadvantages of fungi pretreatment are slow fungal growth (about 2– 4 weeks) and yield loss due to the degradation of polysaccharides along with the degradation of lignin during fungi treatment. To overcome these problems, it seemed to be of interest to use isolated enzymes. One of the advantages of enzymatic treatment is that it takes only a few hours, which is relatively short compare to several days required by fungal treatment. This makes biopulping industrially and economically feasible. Several studies were performed to determine the feasibility of enzymes in a pulping pretreatment and the effects of this treatment on final pulp properties [13–15]. It was shown that the enzyme pretreatment enhanced conventional chemical pulping, increased the extent of delignification as indicated by a reduction in
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Zhao et al. / Enzyme and Microbial Technology 30 (2002) 734 –740
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Table 1 Conditions of X, H, and P bleaching stage Stage
Consistency %
Time min
Temperature °C
Chemical
Charge*
pH value
X
10
120
48
4 IU/g
4.8
H P
6 12
120 120
38–40 80
Xylanase An-76 NaOCl H2O2
7% 1%†
— 10.5–11.5
* Based on weight of oven dry pulp in X, H, P stage on xylanase activity in X stage and on effective chlorine in H stage. ‡ Other conditions: 3% NaSiO3, 0.05% MgSO4, pH value was adjusted with H2SO4 and NaOH.
the kappa number, and improved pulp properties. The enzymes used in biopulping including cellulase, hemicellulase, lignolytic enzymes such as lignin peroxidase (LiP) and manganese peroxidase (MnP), and pectinase etc. Previous research has been focused on the use of enzymes on softwood or hardwood, but not on nonwoody materials, particularly wheat straw with xylanase. The objectives of this work are to investigate the potential of xylanase pretreatment for enhancing conventional soda pulping of wheat straw, and to evaluate the effect of xylanase pretreatment prior to chemical pulping on soda pulping. Changes in the chemical components of wheat straw, crystallinity of cellulose, fiber average lengths and fines content of soda pulps were researched in order to interpret the differences of pulping results from the xylanase pretreated wheat straw and the control.
2. Materials and methods 2.1. Materials Wheat straw collected in Dingtao county (Shandong Province, China) was chopped to 3–5 cm and used to investigate the effects of xylanase pretreatment conditions (i.e. xylanase dosage, pretreatment time, and washing methods of pretreated wheat straw) on pulpability. Wheat straw supplied by Dongying Paper Mill in Shandong Province, China was used in other experiments. A crude enzyme with high xylanase activity and low CMCase (Carboxymethyl cellulase) activity was prepared from culture broth of Aspergillus niger An-76. The medium
contains 2% wheat bran, 0.1% peptone, 0.3% KH2PO4, 0.05% MgSO4, and 1% (NH4)2SO4, and initial pH 6.0. Cultivation was carried out at 30°C for 72 h with a shaking speed of 150 rpm. The supernatant of culture broth was used as a crude enzyme. 2.2. Assays of enzyme Xylanase activity was determined by incubating 0.5 ml suitably diluted enzyme with 1 ml of 1% xylan (oat spelts xylan, Sigma Chemical Company) in a 0.2 M, pH 4.8 acetate buffer for 30 min at 50°C. The activity was expressed as equivalent of reducing sugar produced, which was assayed by DNS (3, 5-dinitrosalicylic acid) reagent [12]. Cellulase activity was measured in a similar manner using carboxymethyl cellulose as substrate. One unit (IU) was defined as the amount (1 mol) of xylose or glucose produced by enzymes per minute under the given conditions. Xylanase activity and cellulase activity of the enzyme An-76 used in the experiments was determined at 357 IU/ml and 2.04 IU/ml according to the methods described above, respectively. 2.3. Enzymatic pretreatment Prior to pulping, wheat straw was treated with the crude enzyme An-76. Wheat straw, enzyme solution and distilled water were mixed thoroughly in polyethylene bags, then the bags were immersed in a constant-temperature (48°C) water bath, and the ratio of liquor to ovendry weight of wheat straw was 7:1 (V/W). The initial pH value (pH 5.0) was adjusted with an acetate buffer (200 mM). After enzyme
Table 2 Effects of xylanase dosages on results of wheat straw soda-pulping*** Xylanase dosage*, IU/g
0
1
2
4
8
Screened-pulp yields, % Kappa number Kappa decreased, % Rejects*, % Residual NaOH, g/l Consumption of alkali**%
40.06 ⫾ 0.23 19.68 ⫾ 0.17 — 0.90 ⫾ 0.21 5.18 ⫾ 0.17 79.28
39.49 ⫾ 0.15 18.82 ⫾ 0.20 4.37 0.81 ⫾ 0.19 5.74 ⫾ 0.21 77.04
39.18 ⫾ 0.21 18.58 ⫾ 0.28 5.59 0.71 ⫾ 0.24 5.70 ⫾ 0.18 77.2
39.99 ⫾ 0.17 18.28 ⫾ 0.17 7.11 0.53 ⫾ 0.07 5.52 ⫾ 0.17 77.92
39.89 ⫾ 0.09 18.11 ⫾ 0.18 7.98 0.46 ⫾ 0.11 5.49 ⫾ 0.21 78.04
* Based on oven dry wheat straw weight. ** Based on alkali charge (15% NaOH) in the cooking stage. *** Wheat straw was treated for 8 hrs in enzyme pretreatment stage, then washed thoroughly with tap water.
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pretreatment, wheat straw was washed with tap water. The controls were prepared as previously described, but without enzyme addition.
3. Results and discussion 3.1. Effects of xylanase pretreatment on wheat straw soda pulping
2.4. Chemical pulping In general, the main operational conditions which affect the final results of soda pulping in enzymatic pretreatment process are the enzyme dosage, temperature, pretreatment time, pH value, and washing methods of xylanase treated wheat straw, etc. It was indicated that the enzyme An-76 was stable in the range of pH 4.0 –5.6 and temperature 45°C-55°C, but its optimum pH and temperature for xylanase activity were pH 5.0 and 48°C according to enzyme properties analysis [18]. Therefore, pH 5.0 and 48°C were selected and used in the xylanase pretreatment stage, and the effects of other pretreatment conditions including enzyme dosage, pretreatment time, and washing methods on soda pulping were studied.
All pulping experiments were performed in a thermoelectrical rotating digester using the following conditions: alkali charge variable, the maximum temperature 160°C, the ratio of liquor to wheat straw 6:1 (V/W), time to maximum temperature 90 min, and time at maximum temperature 30 min. After cooking, the residual NaOH contents in spent black liquor were determined, and pulps were washed thoroughly with tap water and screened with a laboratory flat screen. 2.5. Bleaching of pulps Pulps were bleached in polyethylene bags with HP and XHP bleaching sequence. X, H and P stand for xylanase pretreatment stage, hypochlorite stage and hydrogen peroxide stage, respectively. The bleaching conditions of X, H, and P stage were listed in Table 1. After each stage of bleaching, the pulp was washed with tap water.
3.1.1. Xylanase dosage The effects of xylanase dosages on results of wheat straw soda pulping were shown in Table 2. When compared with the control soda pulps, the xylanase pretreatment resulted in lower kappa number and rejects of pulps, and higher residual NaOH consistency under identical cooking conditions. For example, comparing with the control, pretreatment with 4 IU/g of xylanase can decrease the pulp kappa number from 19.68 to 18.28 (decreased by 7.11%) and rejects from 0.9% to 0.53%, respectively, but the residual NaOH content was increased from 5.18 g/liter to 5.52 g/liter. The yields of screened pulps were comparable for all cooks. This means that the xylanase pretreatment can not only improve pulpability of wheat straw and soda-pulps uniformity, but also decrease the consumption of active alkali during soda cooking. That is to say, treating wheat straw with xylanase prior to soda pulping could reduce the active alkali charge required to attain a same level of delignification. Table 2 shows that the kappa number and rejects of chemical pulps decreased with the increase of the xylanase dosage when all other conditions kept on constant. However, changes in the kappa number and rejects were not significant when xylanase dosage was over 4 IU/g. Therefor, the optimal xylanase dosage was 4 IU/g in enzymatic pretreatment stage.
2.6. Analytical methods The screened yields, kappa number, rejects and physical properties of pulps and the residual NaOH contents were measured according to the Chinese National Standards [16]. The pulp brightness of ISO was measured on a YQ-Z-48A brightness color tester according to the TAPPI standards [17]. Air-dried samples of the xylanase pretreated wheat straw and the control were milled in a laboratory disintegrator to produce straw powders. The powders passing through 40 mesh and retained over 60 mesh were analyzed, viz. hot water extractive, 1% NaOH extractive, alcohol-benzene extractive, Klason lignin, pentosans, and holocellulose, according to the TAPPI standard methods [17]. Crystallinity of cellulose was determined by the X-ray diffraction spectrum method with Rigakv D/max-1200 diffractometer, made in Japan, and fiber average length and fines content were determined with Kajaanii FS-100 Fiber Quality Analyzer (Finland). Table 3 Effect of pretreatment time on results of soda pulping Pretreatment time, hour
4
8
16
24
Screened-pulp yields, % Kappa number Rejects, % Residual NaOH, g/l Consumption of alkali, %
39.57 ⫾ 0.31 18.84 ⫾ 0.22 0.99 ⫾ 0.10 6.12 ⫾ 0.18 75.52
39.18 ⫾ 0.21 18.58 ⫾ 0.28 0.71 ⫾ 0.24 5.70 ⫾ 0.18 77.20
38.87 ⫾ 0.31 18.36 ⫾ 0.29 0.72 ⫾ 0.17 5.68 ⫾ 0.20 77.28
38.54 ⫾ 0.22 18.10 ⫾ 0.22 0.94 ⫾ 0.18 5.53 ⫾ 0.15 77.88
* Xylanase dosage: 2 IU/g, others were shown in Table 2.
Zhao et al. / Enzyme and Microbial Technology 30 (2002) 734 –740
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Table 4 Effects of washing methods on results of soda-pulping
Washing method
Control
Soda pulping after xylanase pretreatment*
Thorough
Thorough
Screened-pulp yields, % 40.06 ⫾ 0.23 39.18 ⫾ 0.21 Kappa number 19.68 ⫾ 0.17 18.58 ⫾ 0.28 Rejects, % 0.90 ⫾ 0.21 0.71 ⫾ 0.24 Residual NaOH, g/l 5.18 ⫾ 0.17 5.70 ⫾ 0.18 Consumption of alkali, % 79.28 77.20
Slight 38.07 ⫾ 0.17 21.62 ⫾ 0.22 1.09 ⫾ 0.15 5.09 ⫾ 0.11 79.64
* Xylanase dosage: 2 IU/g and pretreatment time 8 hours, others were shown in Table 2.
3.1.2. Pretreatment time From Table 3, it can be seen that pretreatment time had no significant effect on soda pulping when time was varied within the range of 4 –24 h. When pretreatment time increase, the kappa number of pulps decreased slightly, and the consumption of active alkali during cooking increased. The pretreatment time of 6 – 8 h was suitable for comprehensive consideration. 3.1.3. Washing methods of pretreated wheat straw After treating wheat straw with xylanase, the pretreated samples were washed thoroughly or slightly in order to investigate the effects of washing methods on final pulp characteristics. Table 4 shows that washing the sample thoroughly with water after enzymatic pretreatment was necessary for improving pulpability of wheat straw. Compared with the pulp from thoroughly washed sample, slight washing method resulted in a higher kappa number and rejects (from 18.58 to 21.68 and from 0.71% to 1.09%, respectively), and a lower yield of screened pulp and residual NaOH consistency (from 39.18% to 38.07% and from 5.70 g/liter to 5.09 g/liter, respectively). Possible reasons for this could be as follows: [1] some water-soluble products with low-
Fig. 1. Effect of xylanase treatment on kappa number of pulps.
Fig. 2. Effect of xylanase treatment on rejects of pulps.
molecular-weight resulting from partial hydrolysis of xylan in enzymatic pretreatment stage could not be dissolved effectively by slight washing method, thus, the diffusion and porosity in sample were not increased. It was not favorable to impregnation of chemicals into wheat straw and removal of dissolved lignin during subsequent chemical cooking. [2] Some acidic components such as glucuronic acid and alkali-soluble degradation products such as oligosaccharide resulting from hemicellulose enzymatic hydrolysis could consume active alkali (NaOH) in cooking liquor, which led to active alkali used for the delignification during chemical cooking decreased. Based on the results of above experiments, the suitable pretreatment conditions with enzyme An-76 were xylanase dosage of 4 IU/g (on oven dry wheat straw weight), 48°C, 6 – 8 h, and pH 5.0. After pretreatment, the pretreated wheat straw should be thoroughly washed.
Fig. 3. Effect of xylanase treatment on residual NaOH.
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a similar influence on soda pulping with different alkali charge. From the above experiments results, it was concluded that the xylanase pretreatment can improve pulpability of wheat straw, decrease kappa number and rejects of pulp at a similar yield of screened pulp, and reduce active alkali consumption during chemical cooking. This may be due to the following reasons. First, some water-soluble products with low molecular weight from partial degradation and hydrolysis of hemicellulose in the xylanase treatment stage were dissolved during washing. It increases porosity in wheat straw, and facilitates the diffusion and impregnation of sodium hydroxide into wheat straw sample and the extraction of degraded-lignin. Thus the xylanase pretreatment can enhance soda pulping and pulp uniformity. The increased diffusivity of sodium hydroxide in pretreated wood with hemicellulase/cellulase solution has been observed by Jacobs-Young et al. [15]. Secondly, parts of extractives in wheat straw and the products from the degradation of carbohydrates were dissolved during enzyme pretreatment and subsequent washing. Thus the dosage of sodium hydroxide consumed for the dissolution of these substances was reduced, relatively, the alkali charge used for the delignification in cooking stage increased. Finally, the xylanase pretreatment can partially degrade and remove xylan from the lignin-carbohydrate-complexes (LCC) by enzyme attacks lignin-carbohydrates linkages such as ether or glycosidic linkages. It is favorable to increasing porosity in wheat straw and solubility of lignin.
Fig. 4. Effect of xylanase treatment on screened pulp yield.
3.2. Effects of xylanase pretreatment on soda-pulping with different alkali charge Wheat straw was treated with xylanase An-76 at the suitable conditions, then cooked with different alkali charge by soda pulping process. It can be seen that, compared with the control pulps, the kappa number of pulps from the xylanase pretreated wheat straw was about 2 units lower (Fig. 1), the rejects decreased (Fig. 2), and the residual NaOH consistency in the black liquor increased, means that the consumption of active alkali reduced under identical cooking condition (Fig. 3). However, the yields of screened pulps had little difference between the xylanase pretreated wheat straw soda pulps and the control pulps (Fig. 4). Thus, the xylanase pretreatment can produce the low kappa number of pulp at identical alkali charge, or achieve the same amount of delignification with less alkali at a slight high yield of screened pulp. For example, when kappa number of pulp was about 14.5, the alkali charge required was 8% for the xylanase pretreated wheat straw and 10% for the control, and the yield of screened pulp was 36.41% and 36.15%, respectively. Fig. 1 and Fig. 3 shown that the change trends of the pulp kappa number and residual NaOH consistency were similar for the xylanase-pretreated samples and the controls, which means the xylanase pretreatment at identical conditions had
3.3. Effects of xylanase pretreatment on physical properties of unbleached pulps Table 5 shows the effects of xylanase pretreatment on the physical properties of unbleached pulps. Compared with the controls, the pulps from wheat straw pretreated with xylanase had higher ISO brightness (about 2% higher), higher tear index, lower tensile index, and lower burst index under similar beating degree conditions. The tear index of a sheet is influenced mainly by fiber average length. The tensile index and burst index are influenced by both the fiber average length and variations in the level of bonding. By evaluating of the changes in fiber average lengths and fines
Table 5 Effects of xylanase pretreatment on physical properties of unbleached pulps Pulps
Controls
Alkali charge, %
8
10
6
Pulp from xylanase-pretreated wheat straw 8
10
12
Beating degree, °SR Basic weight, g/m2 ISO brightness, % Tensile index, N.m/g Tear index, mN.m2/g Burst index, kPa.m2/g
19.0 ⫾ 0.17 67.1 ⫾ 0.09 37.4 ⫾ 0.37 56.11 ⫾ 1.21 6.56 ⫾ 0.29 3.2 ⫾ 0.16
18.5 ⫾ 0.12 67.5 ⫾ 0.12 40.5 ⫾ 0.34 54.41 ⫾ 0.95 6.41 ⫾ 0.21 3.1 ⫾ 0.18
17.2 ⫾ 0.12 67.6 ⫾ 0.12 36.4 ⫾ 0.37 45.33 ⫾ 1.77 6.98 ⫾ 0.31 2.6 ⫾ 0.12
17.0 ⫾ 0.17 68.5 ⫾ 0.09 39.4 ⫾ 0.31 42.20 ⫾ 1.12 7.30 ⫾ 0.29 2.9 ⫾ 0.18
18.0 ⫾ 0.12 67.8 ⫾ 0.09 42.1 ⫾ 0.27 40.98 ⫾ 1.05 7.11 ⫾ 0.21 2.9 ⫾ 0.14
17.5 ⫾ 0.17 67.1 ⫾ 0.12 44.2 ⫾ 0.24 39.92 ⫾ 1.11 6.92 ⫾ 0.19 2.9 ⫾ 0.21
Zhao et al. / Enzyme and Microbial Technology 30 (2002) 734 –740 Table 6 Changes in the fiber length, fines content and crystallinity of cellulose in pulp
Control (4 hrs) Pretreated (4 hrs) Pretreated (7 hrs)
Average fiber length mm
Fines content (⬍0.2 mm), %
Crystallinity of cellulose
1.38 ⫾ 0.01 1.55 ⫾ 0.01 1.45 ⫾ 0.01
9.74 ⫾ 0.21 9.31 ⫾ 0.19 9.11 ⫾ 0.35
58 60 —
content with Kajaanii FS-100 Fiber Quality Analyzer, it was determined that the pulps from xylanase pretreated wheat straw had longer fiber average lengths and lower fines contents than the control (Table 6). Both of these factors seem to be disadvantageous to fiber bonding in the pulps. Lowering the level of fiber bonding lead to the lower tensile index and burst index. The longer fiber average length leads to higher tear index of a sheet from the xylanase pretreated wheat straw soda pulp. Besides, the pulp brightness was increased by xylanase pretreatment due to the decreasing of residual lignin content in the pulps (Fig. 1). Crystallinity of cellulose is indicative of the physical and chemical properties of cellulose to some extent [19]. The experiment results showed that xylanase pretreatment had a slight influence on crystallinity of cellulose (Table 6), therefore, had little influence on inherent strength of individual fiber, and was favored for maintaining the strength properties of the pulp from xylanase pretreated wheat straw. 3.4. Effect of xylanase pretreatment on bleachability of pulps Pulps were bleached with HP and XHP bleaching sequences and the physical properties of bleached pulps were measured to determine the effects of xylanase pretreatment on the bleachability of pulps and physical properties of bleached pulps. The results listed in Table 7 indicated that the brightness of bleached pulps from the xylanase pretreated wheat straw was about 3% (ISO) higher than that of the controls under identical bleaching conditions. This would be attributed to the lower kappa number of pulp from the xylanase pretreated wheat straw. The ISO brightness of bleached pulp was further increased by adding xylanase treatment stage, for example, from 82.1% with HP sequence
739
to 83.9% with XHP sequence for pretreated wheat straw soda pulp, shows that the bleaching of pulp receiving xylanase treatment was more efficient, resulting in higher brightness. This means that xylanase pretreatment can reduce the dosage of bleaching reagents containing chlorine (such as hypochlorite in the experiment) required to attain same brightness of bleached pulp. The bleached pulp from xylanase-pretreated wheat straw had a higher tear index, a lower tensile index, and a lower burst index compared with the control. 3.5. Effect of xylanase pretreatment on chemical components of wheat straw The results of the chemical analysis (Table 8) revealed that the hot water extractive content and 1% NaOH extractive content increased, and alcohol-benzene extractive content decreased with enzymatic treatment. Increased contents of hot water extractive and 1% NaOH extractive indicated that xylanase treatment was able to depolymerize or degrade the wheat straw components (e.g. hemicellulose), and contribute to increase their solubility. The treatment with crude enzyme would also degrade some resin and fatty compounds of wheat straw, led to a decrease in the alcoholbenzene extractive content. The xylanase pretreatment had a slight influence on contents of klason lignin and cellulose, indicated that the treatment alone could not remove the lignin and depolymerize cellulose of wheat straw effectively. However, the treatment makes pentosans content decrease, indicated that hemicellulose was degraded by xylanase, led to a slight decrease in pulp yields. The results testified to some hypotheses given above, and may be partially interpret differences of pulping results from xylanase pretreated wheat straw and the control.
4. Conclusions This study demonstrates that the xylanase pretreatment can enhance soda pulping of wheat straw and pulp uniformity. The suitable pretreatment conditions with enzyme An-76 were xylanase dosage of 4 IU/g (on oven dry wheat straw), 48°C, 6 – 8 h, and pH 5.0, after pretreatment, the pretreated wheat straw was thoroughly washed. Using xy-
Table 7 Bleachability and the physical properties of bleached pulps Pulps*
Bleaching sequence
Brightness % ISO
Beating degree °SR
Tensile index N.m/g
Tear index mN.m2/g
Burst index kPa.m2/g
Control
HP XHP HP XHP
78.7 ⫾ 0.22 80.7 ⫾ 0.34 82.1 ⫾ 0.37 83.9 ⫾ 0.14
20.5 ⫾ 0.17 19.5 ⫾ 0.17 19.0 ⫾ 0.12 18.5 ⫾ 0.12
64.15 ⫾ 1.21 62.36 ⫾ 1.17 58.65 ⫾ 1.12 56.61 ⫾ 1.21
5.52 ⫾ 0.21 6.29 ⫾ 0.19 6.58 ⫾ 0.19 7.02 ⫾ 0.29
3.9 ⫾ 0.16 3.8 ⫾ 0.18 3.7 ⫾ 0.17 3.5 ⫾ 0.14
Pretreated
* Alkali charge of 8% was used in cooking stage.
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Zhao et al. / Enzyme and Microbial Technology 30 (2002) 734 –740
Table 8 Changes in chemical components of wheat straw by xylanase treatment Extractives %
Control Pretreated (4 hrs) Pretreated (7 hrs)
Hot water
1% NaOH
Alcohol
3.81 ⫾ 0.01 3.95 ⫾ 0.05 3.99 ⫾ 0.05
34.45 ⫾ 0.11 35.43 ⫾ 0.09 35.87 ⫾ 0.09
1.93 ⫾ 0.07 1.89 ⫾ 0.07 1.78 ⫾ 0.09
Klason lignin %
Pentosans %
Holocellulose %
Cellulose %
17.34 ⫾ 0.09 17.32 ⫾ 0.11 17.30 ⫾ 0.12
26.20 ⫾ 0.12 25.36 ⫾ 0.11 25.30 ⫾ 0.15
80.18 ⫾ 0.17 79.33 ⫾ 0.17 78.48 ⫾ 0.19
53.98 53.97 53.18
* Percentage based on ovendry wheat straw weight. ** Cellulose % ⫽ holocellulose % ⫺ pentosans %
lanase treatment prior to chemical cooking, the pulp with a similar screened pulp yield, a lower kappa number, and lower rejects could be produced under identical cooking conditions, and the consumption of active alkali was reduced during cooking. Bleached pulp from the xylanase pretreated wheat straw had higher ISO brightness than the controls using HP and XHP short sequence bleaching. All the unbleached pulps and bleached pulps from xylanasepretreated wheat straw had a higher tear index, a lower tensile index, and a lower burst index. Xylanase treatment can increase the contents of hot water extractive and 1% NaOH extractive, and decrease the alcohol-benzene extractive content of wheat straw. The treatment has a slight influence on the contents of klason lignin and cellulose, but decrease the content of pentosan. Enzymatic treatment leads to slight increasing crystallinity of cellulose. The pulps from xylanase pretreated wheat straw had a longer fiber length and a lower fines content compared with the control. The results of chemical analysis may be partially interpret differences of pulping results from xylanase pretreated wheat straw and the control.
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Xylanase pretreatment leads to enhanced soda pulping of wheat straw Jian Zhao,* Xuezhi Li, Yinbo Qu, Peiji Gao State Key Laboratory of Microbial Technology, Shandong University, Jinan 250100, People’s Republic of China Received 15 June 2001; received in revised form 08 November 2001; accepted 26 November 2001
Abstract Wheat straw was treated with a crude enzyme containing mainly xylanase prior to soda cooking. The enzyme was prepared from the culture filtration of Aspergillus niger strain An-76. The suitable conditions of pretreatment were xylanase dosage of 4 IU/g (on oven dry wheat straw), 48°C, 6 – 8 h, and pH 5.0, pretreated wheat straw was thorough washed after pretreatment. Xylanase pretreatment can improve pulpability of wheat straw effectively, and produce the pulps with lower kappa number (about 2 units lower), lower rejects and similar yields of screened-pulps under identical pulping conditions. The residual active alkali consistency of black liquor was increased by xylanase pretreatment. The brightness of bleached pulps from the xylanase pretreated wheat straw was about 3%ISO higher than that of the controls using identical bleaching sequence and conditions. The pulps, including unbleached pulps and bleached pulps, from the xylanase pretreated wheat straw had a higher tear index, a lower tensile index, and a lower burst index as compared to the control. The changes in the chemical components of wheat straw, crystallizability degree of cellulose, fiber average length and fines contents in the pulps were investigated in order to interpret differences of pulping results from the xylanase pretreated wheat straw and the controls. © 2002 Elsevier Science Inc. All rights reserved. Keywords: Xylanase pretreatment; Soda pulping; Aspergillus niger; Wheat straw
1. Introduction At present, wheat straw is an important raw material for pulp and paper manufacture and soda process is one of the main pulping processes to produce chemical pulp from wheat straw in the pulp and paper industry in China. However, the conventional soda process causes a serious wastewater pollution problem. In recent years, increasing environmental pressures and more rigorous regulations on strengthening the environmental protection have made it necessary to search for alternative methods for reducing wastewater pollution from pulping and bleaching plant in pulping industry. These pressures have also led to the design of many techniques aimed at improving the pulping process and reducing the lignin content of the pulp entering bleach plant. Lowering the kappa number of pulp by increasing the efficiency of the pulping process has led to a reduction in the amount of chemical reagents necessary for pulp bleaching and a concomitant reduction in the pollutants discharged from bleach plant. * Corresponding author. Tel.: ⫹86 531 8564429; fax: ⫹86 531 8565234 E-mail address: [email protected] (J. Zhao).
During the last decade, a large number of studies have been carried out on biopulping and biobleaching, especially biomechanical- and biochemical-pulping of wood (including softwood and hardwood) involving the use of lignin degrading fungi (i.e. white rot fungi) to treat wood chips and the use of xylanases to treat pulps prior to conventional pulping and bleaching [1–12]. Fungi pretreatment might reduce the refining energy and chemicals consumption of pulping processes or improve pulps properties, and reduce the negative environmental impact of pulping. But the main disadvantages of fungi pretreatment are slow fungal growth (about 2– 4 weeks) and yield loss due to the degradation of polysaccharides along with the degradation of lignin during fungi treatment. To overcome these problems, it seemed to be of interest to use isolated enzymes. One of the advantages of enzymatic treatment is that it takes only a few hours, which is relatively short compare to several days required by fungal treatment. This makes biopulping industrially and economically feasible. Several studies were performed to determine the feasibility of enzymes in a pulping pretreatment and the effects of this treatment on final pulp properties [13–15]. It was shown that the enzyme pretreatment enhanced conventional chemical pulping, increased the extent of delignification as indicated by a reduction in
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Zhao et al. / Enzyme and Microbial Technology 30 (2002) 734 –740
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Table 1 Conditions of X, H, and P bleaching stage Stage
Consistency %
Time min
Temperature °C
Chemical
Charge*
pH value
X
10
120
48
4 IU/g
4.8
H P
6 12
120 120
38–40 80
Xylanase An-76 NaOCl H2O2
7% 1%†
— 10.5–11.5
* Based on weight of oven dry pulp in X, H, P stage on xylanase activity in X stage and on effective chlorine in H stage. ‡ Other conditions: 3% NaSiO3, 0.05% MgSO4, pH value was adjusted with H2SO4 and NaOH.
the kappa number, and improved pulp properties. The enzymes used in biopulping including cellulase, hemicellulase, lignolytic enzymes such as lignin peroxidase (LiP) and manganese peroxidase (MnP), and pectinase etc. Previous research has been focused on the use of enzymes on softwood or hardwood, but not on nonwoody materials, particularly wheat straw with xylanase. The objectives of this work are to investigate the potential of xylanase pretreatment for enhancing conventional soda pulping of wheat straw, and to evaluate the effect of xylanase pretreatment prior to chemical pulping on soda pulping. Changes in the chemical components of wheat straw, crystallinity of cellulose, fiber average lengths and fines content of soda pulps were researched in order to interpret the differences of pulping results from the xylanase pretreated wheat straw and the control.
2. Materials and methods 2.1. Materials Wheat straw collected in Dingtao county (Shandong Province, China) was chopped to 3–5 cm and used to investigate the effects of xylanase pretreatment conditions (i.e. xylanase dosage, pretreatment time, and washing methods of pretreated wheat straw) on pulpability. Wheat straw supplied by Dongying Paper Mill in Shandong Province, China was used in other experiments. A crude enzyme with high xylanase activity and low CMCase (Carboxymethyl cellulase) activity was prepared from culture broth of Aspergillus niger An-76. The medium
contains 2% wheat bran, 0.1% peptone, 0.3% KH2PO4, 0.05% MgSO4, and 1% (NH4)2SO4, and initial pH 6.0. Cultivation was carried out at 30°C for 72 h with a shaking speed of 150 rpm. The supernatant of culture broth was used as a crude enzyme. 2.2. Assays of enzyme Xylanase activity was determined by incubating 0.5 ml suitably diluted enzyme with 1 ml of 1% xylan (oat spelts xylan, Sigma Chemical Company) in a 0.2 M, pH 4.8 acetate buffer for 30 min at 50°C. The activity was expressed as equivalent of reducing sugar produced, which was assayed by DNS (3, 5-dinitrosalicylic acid) reagent [12]. Cellulase activity was measured in a similar manner using carboxymethyl cellulose as substrate. One unit (IU) was defined as the amount (1 mol) of xylose or glucose produced by enzymes per minute under the given conditions. Xylanase activity and cellulase activity of the enzyme An-76 used in the experiments was determined at 357 IU/ml and 2.04 IU/ml according to the methods described above, respectively. 2.3. Enzymatic pretreatment Prior to pulping, wheat straw was treated with the crude enzyme An-76. Wheat straw, enzyme solution and distilled water were mixed thoroughly in polyethylene bags, then the bags were immersed in a constant-temperature (48°C) water bath, and the ratio of liquor to ovendry weight of wheat straw was 7:1 (V/W). The initial pH value (pH 5.0) was adjusted with an acetate buffer (200 mM). After enzyme
Table 2 Effects of xylanase dosages on results of wheat straw soda-pulping*** Xylanase dosage*, IU/g
0
1
2
4
8
Screened-pulp yields, % Kappa number Kappa decreased, % Rejects*, % Residual NaOH, g/l Consumption of alkali**%
40.06 ⫾ 0.23 19.68 ⫾ 0.17 — 0.90 ⫾ 0.21 5.18 ⫾ 0.17 79.28
39.49 ⫾ 0.15 18.82 ⫾ 0.20 4.37 0.81 ⫾ 0.19 5.74 ⫾ 0.21 77.04
39.18 ⫾ 0.21 18.58 ⫾ 0.28 5.59 0.71 ⫾ 0.24 5.70 ⫾ 0.18 77.2
39.99 ⫾ 0.17 18.28 ⫾ 0.17 7.11 0.53 ⫾ 0.07 5.52 ⫾ 0.17 77.92
39.89 ⫾ 0.09 18.11 ⫾ 0.18 7.98 0.46 ⫾ 0.11 5.49 ⫾ 0.21 78.04
* Based on oven dry wheat straw weight. ** Based on alkali charge (15% NaOH) in the cooking stage. *** Wheat straw was treated for 8 hrs in enzyme pretreatment stage, then washed thoroughly with tap water.
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pretreatment, wheat straw was washed with tap water. The controls were prepared as previously described, but without enzyme addition.
3. Results and discussion 3.1. Effects of xylanase pretreatment on wheat straw soda pulping
2.4. Chemical pulping In general, the main operational conditions which affect the final results of soda pulping in enzymatic pretreatment process are the enzyme dosage, temperature, pretreatment time, pH value, and washing methods of xylanase treated wheat straw, etc. It was indicated that the enzyme An-76 was stable in the range of pH 4.0 –5.6 and temperature 45°C-55°C, but its optimum pH and temperature for xylanase activity were pH 5.0 and 48°C according to enzyme properties analysis [18]. Therefore, pH 5.0 and 48°C were selected and used in the xylanase pretreatment stage, and the effects of other pretreatment conditions including enzyme dosage, pretreatment time, and washing methods on soda pulping were studied.
All pulping experiments were performed in a thermoelectrical rotating digester using the following conditions: alkali charge variable, the maximum temperature 160°C, the ratio of liquor to wheat straw 6:1 (V/W), time to maximum temperature 90 min, and time at maximum temperature 30 min. After cooking, the residual NaOH contents in spent black liquor were determined, and pulps were washed thoroughly with tap water and screened with a laboratory flat screen. 2.5. Bleaching of pulps Pulps were bleached in polyethylene bags with HP and XHP bleaching sequence. X, H and P stand for xylanase pretreatment stage, hypochlorite stage and hydrogen peroxide stage, respectively. The bleaching conditions of X, H, and P stage were listed in Table 1. After each stage of bleaching, the pulp was washed with tap water.
3.1.1. Xylanase dosage The effects of xylanase dosages on results of wheat straw soda pulping were shown in Table 2. When compared with the control soda pulps, the xylanase pretreatment resulted in lower kappa number and rejects of pulps, and higher residual NaOH consistency under identical cooking conditions. For example, comparing with the control, pretreatment with 4 IU/g of xylanase can decrease the pulp kappa number from 19.68 to 18.28 (decreased by 7.11%) and rejects from 0.9% to 0.53%, respectively, but the residual NaOH content was increased from 5.18 g/liter to 5.52 g/liter. The yields of screened pulps were comparable for all cooks. This means that the xylanase pretreatment can not only improve pulpability of wheat straw and soda-pulps uniformity, but also decrease the consumption of active alkali during soda cooking. That is to say, treating wheat straw with xylanase prior to soda pulping could reduce the active alkali charge required to attain a same level of delignification. Table 2 shows that the kappa number and rejects of chemical pulps decreased with the increase of the xylanase dosage when all other conditions kept on constant. However, changes in the kappa number and rejects were not significant when xylanase dosage was over 4 IU/g. Therefor, the optimal xylanase dosage was 4 IU/g in enzymatic pretreatment stage.
2.6. Analytical methods The screened yields, kappa number, rejects and physical properties of pulps and the residual NaOH contents were measured according to the Chinese National Standards [16]. The pulp brightness of ISO was measured on a YQ-Z-48A brightness color tester according to the TAPPI standards [17]. Air-dried samples of the xylanase pretreated wheat straw and the control were milled in a laboratory disintegrator to produce straw powders. The powders passing through 40 mesh and retained over 60 mesh were analyzed, viz. hot water extractive, 1% NaOH extractive, alcohol-benzene extractive, Klason lignin, pentosans, and holocellulose, according to the TAPPI standard methods [17]. Crystallinity of cellulose was determined by the X-ray diffraction spectrum method with Rigakv D/max-1200 diffractometer, made in Japan, and fiber average length and fines content were determined with Kajaanii FS-100 Fiber Quality Analyzer (Finland). Table 3 Effect of pretreatment time on results of soda pulping Pretreatment time, hour
4
8
16
24
Screened-pulp yields, % Kappa number Rejects, % Residual NaOH, g/l Consumption of alkali, %
39.57 ⫾ 0.31 18.84 ⫾ 0.22 0.99 ⫾ 0.10 6.12 ⫾ 0.18 75.52
39.18 ⫾ 0.21 18.58 ⫾ 0.28 0.71 ⫾ 0.24 5.70 ⫾ 0.18 77.20
38.87 ⫾ 0.31 18.36 ⫾ 0.29 0.72 ⫾ 0.17 5.68 ⫾ 0.20 77.28
38.54 ⫾ 0.22 18.10 ⫾ 0.22 0.94 ⫾ 0.18 5.53 ⫾ 0.15 77.88
* Xylanase dosage: 2 IU/g, others were shown in Table 2.
Zhao et al. / Enzyme and Microbial Technology 30 (2002) 734 –740
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Table 4 Effects of washing methods on results of soda-pulping
Washing method
Control
Soda pulping after xylanase pretreatment*
Thorough
Thorough
Screened-pulp yields, % 40.06 ⫾ 0.23 39.18 ⫾ 0.21 Kappa number 19.68 ⫾ 0.17 18.58 ⫾ 0.28 Rejects, % 0.90 ⫾ 0.21 0.71 ⫾ 0.24 Residual NaOH, g/l 5.18 ⫾ 0.17 5.70 ⫾ 0.18 Consumption of alkali, % 79.28 77.20
Slight 38.07 ⫾ 0.17 21.62 ⫾ 0.22 1.09 ⫾ 0.15 5.09 ⫾ 0.11 79.64
* Xylanase dosage: 2 IU/g and pretreatment time 8 hours, others were shown in Table 2.
3.1.2. Pretreatment time From Table 3, it can be seen that pretreatment time had no significant effect on soda pulping when time was varied within the range of 4 –24 h. When pretreatment time increase, the kappa number of pulps decreased slightly, and the consumption of active alkali during cooking increased. The pretreatment time of 6 – 8 h was suitable for comprehensive consideration. 3.1.3. Washing methods of pretreated wheat straw After treating wheat straw with xylanase, the pretreated samples were washed thoroughly or slightly in order to investigate the effects of washing methods on final pulp characteristics. Table 4 shows that washing the sample thoroughly with water after enzymatic pretreatment was necessary for improving pulpability of wheat straw. Compared with the pulp from thoroughly washed sample, slight washing method resulted in a higher kappa number and rejects (from 18.58 to 21.68 and from 0.71% to 1.09%, respectively), and a lower yield of screened pulp and residual NaOH consistency (from 39.18% to 38.07% and from 5.70 g/liter to 5.09 g/liter, respectively). Possible reasons for this could be as follows: [1] some water-soluble products with low-
Fig. 1. Effect of xylanase treatment on kappa number of pulps.
Fig. 2. Effect of xylanase treatment on rejects of pulps.
molecular-weight resulting from partial hydrolysis of xylan in enzymatic pretreatment stage could not be dissolved effectively by slight washing method, thus, the diffusion and porosity in sample were not increased. It was not favorable to impregnation of chemicals into wheat straw and removal of dissolved lignin during subsequent chemical cooking. [2] Some acidic components such as glucuronic acid and alkali-soluble degradation products such as oligosaccharide resulting from hemicellulose enzymatic hydrolysis could consume active alkali (NaOH) in cooking liquor, which led to active alkali used for the delignification during chemical cooking decreased. Based on the results of above experiments, the suitable pretreatment conditions with enzyme An-76 were xylanase dosage of 4 IU/g (on oven dry wheat straw weight), 48°C, 6 – 8 h, and pH 5.0. After pretreatment, the pretreated wheat straw should be thoroughly washed.
Fig. 3. Effect of xylanase treatment on residual NaOH.
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a similar influence on soda pulping with different alkali charge. From the above experiments results, it was concluded that the xylanase pretreatment can improve pulpability of wheat straw, decrease kappa number and rejects of pulp at a similar yield of screened pulp, and reduce active alkali consumption during chemical cooking. This may be due to the following reasons. First, some water-soluble products with low molecular weight from partial degradation and hydrolysis of hemicellulose in the xylanase treatment stage were dissolved during washing. It increases porosity in wheat straw, and facilitates the diffusion and impregnation of sodium hydroxide into wheat straw sample and the extraction of degraded-lignin. Thus the xylanase pretreatment can enhance soda pulping and pulp uniformity. The increased diffusivity of sodium hydroxide in pretreated wood with hemicellulase/cellulase solution has been observed by Jacobs-Young et al. [15]. Secondly, parts of extractives in wheat straw and the products from the degradation of carbohydrates were dissolved during enzyme pretreatment and subsequent washing. Thus the dosage of sodium hydroxide consumed for the dissolution of these substances was reduced, relatively, the alkali charge used for the delignification in cooking stage increased. Finally, the xylanase pretreatment can partially degrade and remove xylan from the lignin-carbohydrate-complexes (LCC) by enzyme attacks lignin-carbohydrates linkages such as ether or glycosidic linkages. It is favorable to increasing porosity in wheat straw and solubility of lignin.
Fig. 4. Effect of xylanase treatment on screened pulp yield.
3.2. Effects of xylanase pretreatment on soda-pulping with different alkali charge Wheat straw was treated with xylanase An-76 at the suitable conditions, then cooked with different alkali charge by soda pulping process. It can be seen that, compared with the control pulps, the kappa number of pulps from the xylanase pretreated wheat straw was about 2 units lower (Fig. 1), the rejects decreased (Fig. 2), and the residual NaOH consistency in the black liquor increased, means that the consumption of active alkali reduced under identical cooking condition (Fig. 3). However, the yields of screened pulps had little difference between the xylanase pretreated wheat straw soda pulps and the control pulps (Fig. 4). Thus, the xylanase pretreatment can produce the low kappa number of pulp at identical alkali charge, or achieve the same amount of delignification with less alkali at a slight high yield of screened pulp. For example, when kappa number of pulp was about 14.5, the alkali charge required was 8% for the xylanase pretreated wheat straw and 10% for the control, and the yield of screened pulp was 36.41% and 36.15%, respectively. Fig. 1 and Fig. 3 shown that the change trends of the pulp kappa number and residual NaOH consistency were similar for the xylanase-pretreated samples and the controls, which means the xylanase pretreatment at identical conditions had
3.3. Effects of xylanase pretreatment on physical properties of unbleached pulps Table 5 shows the effects of xylanase pretreatment on the physical properties of unbleached pulps. Compared with the controls, the pulps from wheat straw pretreated with xylanase had higher ISO brightness (about 2% higher), higher tear index, lower tensile index, and lower burst index under similar beating degree conditions. The tear index of a sheet is influenced mainly by fiber average length. The tensile index and burst index are influenced by both the fiber average length and variations in the level of bonding. By evaluating of the changes in fiber average lengths and fines
Table 5 Effects of xylanase pretreatment on physical properties of unbleached pulps Pulps
Controls
Alkali charge, %
8
10
6
Pulp from xylanase-pretreated wheat straw 8
10
12
Beating degree, °SR Basic weight, g/m2 ISO brightness, % Tensile index, N.m/g Tear index, mN.m2/g Burst index, kPa.m2/g
19.0 ⫾ 0.17 67.1 ⫾ 0.09 37.4 ⫾ 0.37 56.11 ⫾ 1.21 6.56 ⫾ 0.29 3.2 ⫾ 0.16
18.5 ⫾ 0.12 67.5 ⫾ 0.12 40.5 ⫾ 0.34 54.41 ⫾ 0.95 6.41 ⫾ 0.21 3.1 ⫾ 0.18
17.2 ⫾ 0.12 67.6 ⫾ 0.12 36.4 ⫾ 0.37 45.33 ⫾ 1.77 6.98 ⫾ 0.31 2.6 ⫾ 0.12
17.0 ⫾ 0.17 68.5 ⫾ 0.09 39.4 ⫾ 0.31 42.20 ⫾ 1.12 7.30 ⫾ 0.29 2.9 ⫾ 0.18
18.0 ⫾ 0.12 67.8 ⫾ 0.09 42.1 ⫾ 0.27 40.98 ⫾ 1.05 7.11 ⫾ 0.21 2.9 ⫾ 0.14
17.5 ⫾ 0.17 67.1 ⫾ 0.12 44.2 ⫾ 0.24 39.92 ⫾ 1.11 6.92 ⫾ 0.19 2.9 ⫾ 0.21
Zhao et al. / Enzyme and Microbial Technology 30 (2002) 734 –740 Table 6 Changes in the fiber length, fines content and crystallinity of cellulose in pulp
Control (4 hrs) Pretreated (4 hrs) Pretreated (7 hrs)
Average fiber length mm
Fines content (⬍0.2 mm), %
Crystallinity of cellulose
1.38 ⫾ 0.01 1.55 ⫾ 0.01 1.45 ⫾ 0.01
9.74 ⫾ 0.21 9.31 ⫾ 0.19 9.11 ⫾ 0.35
58 60 —
content with Kajaanii FS-100 Fiber Quality Analyzer, it was determined that the pulps from xylanase pretreated wheat straw had longer fiber average lengths and lower fines contents than the control (Table 6). Both of these factors seem to be disadvantageous to fiber bonding in the pulps. Lowering the level of fiber bonding lead to the lower tensile index and burst index. The longer fiber average length leads to higher tear index of a sheet from the xylanase pretreated wheat straw soda pulp. Besides, the pulp brightness was increased by xylanase pretreatment due to the decreasing of residual lignin content in the pulps (Fig. 1). Crystallinity of cellulose is indicative of the physical and chemical properties of cellulose to some extent [19]. The experiment results showed that xylanase pretreatment had a slight influence on crystallinity of cellulose (Table 6), therefore, had little influence on inherent strength of individual fiber, and was favored for maintaining the strength properties of the pulp from xylanase pretreated wheat straw. 3.4. Effect of xylanase pretreatment on bleachability of pulps Pulps were bleached with HP and XHP bleaching sequences and the physical properties of bleached pulps were measured to determine the effects of xylanase pretreatment on the bleachability of pulps and physical properties of bleached pulps. The results listed in Table 7 indicated that the brightness of bleached pulps from the xylanase pretreated wheat straw was about 3% (ISO) higher than that of the controls under identical bleaching conditions. This would be attributed to the lower kappa number of pulp from the xylanase pretreated wheat straw. The ISO brightness of bleached pulp was further increased by adding xylanase treatment stage, for example, from 82.1% with HP sequence
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to 83.9% with XHP sequence for pretreated wheat straw soda pulp, shows that the bleaching of pulp receiving xylanase treatment was more efficient, resulting in higher brightness. This means that xylanase pretreatment can reduce the dosage of bleaching reagents containing chlorine (such as hypochlorite in the experiment) required to attain same brightness of bleached pulp. The bleached pulp from xylanase-pretreated wheat straw had a higher tear index, a lower tensile index, and a lower burst index compared with the control. 3.5. Effect of xylanase pretreatment on chemical components of wheat straw The results of the chemical analysis (Table 8) revealed that the hot water extractive content and 1% NaOH extractive content increased, and alcohol-benzene extractive content decreased with enzymatic treatment. Increased contents of hot water extractive and 1% NaOH extractive indicated that xylanase treatment was able to depolymerize or degrade the wheat straw components (e.g. hemicellulose), and contribute to increase their solubility. The treatment with crude enzyme would also degrade some resin and fatty compounds of wheat straw, led to a decrease in the alcoholbenzene extractive content. The xylanase pretreatment had a slight influence on contents of klason lignin and cellulose, indicated that the treatment alone could not remove the lignin and depolymerize cellulose of wheat straw effectively. However, the treatment makes pentosans content decrease, indicated that hemicellulose was degraded by xylanase, led to a slight decrease in pulp yields. The results testified to some hypotheses given above, and may be partially interpret differences of pulping results from xylanase pretreated wheat straw and the control.
4. Conclusions This study demonstrates that the xylanase pretreatment can enhance soda pulping of wheat straw and pulp uniformity. The suitable pretreatment conditions with enzyme An-76 were xylanase dosage of 4 IU/g (on oven dry wheat straw), 48°C, 6 – 8 h, and pH 5.0, after pretreatment, the pretreated wheat straw was thoroughly washed. Using xy-
Table 7 Bleachability and the physical properties of bleached pulps Pulps*
Bleaching sequence
Brightness % ISO
Beating degree °SR
Tensile index N.m/g
Tear index mN.m2/g
Burst index kPa.m2/g
Control
HP XHP HP XHP
78.7 ⫾ 0.22 80.7 ⫾ 0.34 82.1 ⫾ 0.37 83.9 ⫾ 0.14
20.5 ⫾ 0.17 19.5 ⫾ 0.17 19.0 ⫾ 0.12 18.5 ⫾ 0.12
64.15 ⫾ 1.21 62.36 ⫾ 1.17 58.65 ⫾ 1.12 56.61 ⫾ 1.21
5.52 ⫾ 0.21 6.29 ⫾ 0.19 6.58 ⫾ 0.19 7.02 ⫾ 0.29
3.9 ⫾ 0.16 3.8 ⫾ 0.18 3.7 ⫾ 0.17 3.5 ⫾ 0.14
Pretreated
* Alkali charge of 8% was used in cooking stage.
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Table 8 Changes in chemical components of wheat straw by xylanase treatment Extractives %
Control Pretreated (4 hrs) Pretreated (7 hrs)
Hot water
1% NaOH
Alcohol
3.81 ⫾ 0.01 3.95 ⫾ 0.05 3.99 ⫾ 0.05
34.45 ⫾ 0.11 35.43 ⫾ 0.09 35.87 ⫾ 0.09
1.93 ⫾ 0.07 1.89 ⫾ 0.07 1.78 ⫾ 0.09
Klason lignin %
Pentosans %
Holocellulose %
Cellulose %
17.34 ⫾ 0.09 17.32 ⫾ 0.11 17.30 ⫾ 0.12
26.20 ⫾ 0.12 25.36 ⫾ 0.11 25.30 ⫾ 0.15
80.18 ⫾ 0.17 79.33 ⫾ 0.17 78.48 ⫾ 0.19
53.98 53.97 53.18
* Percentage based on ovendry wheat straw weight. ** Cellulose % ⫽ holocellulose % ⫺ pentosans %
lanase treatment prior to chemical cooking, the pulp with a similar screened pulp yield, a lower kappa number, and lower rejects could be produced under identical cooking conditions, and the consumption of active alkali was reduced during cooking. Bleached pulp from the xylanase pretreated wheat straw had higher ISO brightness than the controls using HP and XHP short sequence bleaching. All the unbleached pulps and bleached pulps from xylanasepretreated wheat straw had a higher tear index, a lower tensile index, and a lower burst index. Xylanase treatment can increase the contents of hot water extractive and 1% NaOH extractive, and decrease the alcohol-benzene extractive content of wheat straw. The treatment has a slight influence on the contents of klason lignin and cellulose, but decrease the content of pentosan. Enzymatic treatment leads to slight increasing crystallinity of cellulose. The pulps from xylanase pretreated wheat straw had a longer fiber length and a lower fines content compared with the control. The results of chemical analysis may be partially interpret differences of pulping results from xylanase pretreated wheat straw and the control.
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